I wish I knew. I've never been in a position to go away for more than about 9 days, and that was only for overseas trips where I was visiting family.
I think it would depend a lot on the destination and my level of comfort with my lodging. The longer the trip the more I like having a more home-like ambience around me. I'm not a huge fan of moving every couple of days; I prefer to pick one base or two at most and stay within day-trip distance of them. (A cruise with lots of ports would probably be different, as the ship remains the same home-base for the duration.)
My work goes where I go these days, so there's no escaping it for a long stretch of time. What I find is most likely to make me itch to go home is not being comfortable in my lodging. A while back we stayed in a beach condo that had a mattress that was *rock*-hard; it was very difficult to enjoy ourselves knowing that we had to suffer that discomfort every night.
Over time our vacations have gotten longer over time but they are also situational.
Most often if it's a normal domestic or international beach vacation 8-10 days is good.
Vegas is 4 days 3 nights is our sweet spot
Skiing trips are usually 3 days or so
Our cruise was 14 days as it included transit there and back.
Our upcoming Europe trip is hovering around 15-16 days as it includes transit there and back (and includes additional nights being spent on the East Coast before getting back home), not sure how long are Japan trip will be but probably similar as we plan on staying overnight on the West Coast.
WDW usually is 10-11 days includes Universal and transit days.
We used to do weekend B&B style stuff every now and then that would be basically 3 days 2 nights but haven't done that in a long while.
We found 5 days was too much, 7 def. too much although that was when my husband was doing a work conference for multiple days but we had all the evenings available and with Vegas that's where a lot of the action is at. 3 days 2 nights and we don't feel like we got enough time so 4 days 3 nights seems to be perfect for us.Four days in Vegas is an eternity. Usually that's how many days I need before I max out. We just went a couple of weeks ago and stayed for the first time at the Wynn. Initially we were at an MGM resort but they had a cyber attack so we moved to the Wynn. Loved it! Could have stayed for a week between the pool during the day and the piano bar in the evening. I'm too cheap to gamble there. LOL. It felt very resort like with lots of landscaping and theming.
I can't do more than a few days in Vegas too. I go every year for a conference after day 3 or 4, I'm ready to come home. And I'm way to cheap to gamble too, maybe sometimes bingo.Four days in Vegas is an eternity. Usually that's how many days I need before I max out. We just went a couple of weeks ago and stayed for the first time at the Wynn. Initially we were at an MGM resort but they had a cyber attack so we moved to the Wynn. Loved it! Could have stayed for a week between the pool during the day and the piano bar in the evening. I'm too cheap to gamble there. LOL. It felt very resort like with lots of landscaping and theming.